It is well known that modern manufacturing techniques deploy a number of portable tools to tackle the repetitive jobs that are encountered in the shop floor of the industry. Tools like drills, grinders, shears, nibblers, screw drivers, nut runners and impact wrenches find extensive use in large fabrication shops, tool rooms, fettling shops and assembly lines. In addition such tools also find large usage as do-it-yourself (DIY) tools in the hands of the individuals in domestic applications. Also such portable tools are widely used in mines particularly in the form of drills. The main requirements of such tools are portability, ease of handling and usage, safety, high efficiency and maximum power output The manufacturer always aims to achieve the maximum power-to-weight ratio in such tools. Nowadays the energy consumed by such tools is also becoming an important criterion for selection.
Presently available range of portable tools can be broadly classified into three types. They are given as under:
AC/DC universal motor based electric tools; PA1 pneumatic tools; and PA1 conventional high frequency tools. PA1 The Commutator/Carbon Brush-gear is a perennial source of problem leading to reduced reliability and increased maintenance; PA1 In these motors, the speed drop from no-load to full-load is very high, often of the order of 2:1. Excessive overloads can cause stalling of the motor, leading to armature burnout; PA1 As the motor operates on mains supply, it has to be designed with double-insulation or reinforced insulation with proper earthing, to achieve the required safety levels; and PA1 It is difficult to make these tools flameproof for use in mines and other hazardous areas. PA1 Like universal tools, the pneumatic tools also exhibit steep fall in speed on increasing loads and a tendency for stalling; PA1 They require a centralized compressed-air line, which is expensive and difficult to maintain; PA1 As the pneumatic motor operates best on dry air, free from dust and moisture, each of the tools must be equipped with a FRL (Filter-Regulator-Lubricator) unit; PA1 Over its life, due to continuous ware and tare the pneumatic tool requires regular maintenance, as also the air line; PA1 Even with a well-designed compressor and air line, the pneumatic system is very inefficient. The overall efficiency of the system, as measured by the ratio of the power available at the output shaft of the tool to the input of the motor of the compressor, is very poor compared to the electric system; and PA1 The overall systems costs are quite high. PA1 a casing for a motor; PA1 a non-drive end cover having a bearing at its center for the motor; PA1 a heat sink provided either integrally or separately on the covers; PA1 PWM bridge inverter consisting of power transistors with corresponding gates, the output of the PWM bridge inverter is connected to the said motor; PA1 the power transistors terminals are connected to a printed circuit board (PB) and are mounted on the heat sink; PA1 the controller unit having a software program of short code length and the driver IC for driving the gates are connected to another printed circuit board (CB); PA1 the two boards (PB & CB) are inter-connected for determining the timing sequences for generating the signals for switching ON/OFF the gates of the power transistors of the PWM bridge inverter in order to produce variable voltage variable frequency (VVVF), sinusoidal wave forms for controlling the speed of the said motor using space vector pulse width modulation (SVPWM) or sinusoidal pulse width modulation (SPWM) technique, and are mounted through mounting means to the heat sink; and PA1 a cooling means mounted on the shaft of the motor to first cool the electronics of PB & CB mounted on the heat sink and thereafter cool the stator of the motor; PA1 an input rectifier and the filter capacitors are connected to PWM bridge inverter, an auxiliary power supply, which provides the power supply to the controller unit and the driver IC are housed in the handle of the tool.
The first type mentioned above uses a universal electric motor. These motors are essentially series-wound DC commutator motors that have been specially designed to operate in AC as well. The stator core is invariably laminated to reduce losses. A well-designed universal motor is cheap, lightweight and operatable directly from the AC mains. The fact that it does not require a special power source is a major reason for its widespread use. This type of tool finds extensive use in DIY applications and small shops where one off usage is normal.
The universal electric tools suffer from certain disadvantages in that
Due to these reasons the universal electric tools are generally not preferred for heavy duty, continuous loads and in arduous working environment.
The second type, viz., the pneumatic tools was essentially developed to overcome some of the problems associated with the universal electric tools. The pneumatic tool operates from high-pressure compressed air by means of a simple drive called vane motor. A rotor with vanes supported on bearings runs inside a housing due to the passage of the compressed air and high speeds are achieved. A reduction gearbox is used to reduce the speed and increase the torque. While the pneumatic tool is versatile and absolutely safe, they also suffer from certain disadvantages, as detailed below:
It is mainly to obviate the drawbacks of the universal electric and pneumatic tools that the high frequency (BF) tools were developed. The HF tools employ a three-phase AC induction motor as the prime mover. This motor, with its virtually indestructible die-cast rotor, is very rugged and reliable. Also the motor exhibits a speed-torque characteristic that is totally different from the universal and pneumatic tools. The speed of the induction motor drops very little on the application of the fall load and this results in higher productivity and it is virtually impossible to stall this type of motor by hand.
It is also a known fact that the speed of the induction motor is proportional to the frequency and at the nominal power frequency of 50 or 60Hz that is generally available, the maximum speed that can be achieved from an induction motor is only about 3000/3600 rpm. As the size of an electric motor for a given output is inversely proportional to its operating speed, the size of the motor for a particular output will be higher than that of the universal motor with its high operating speeds. Thus an induction motor operating from the conventional line frequency will be heavy and portability can be achieved only by increasing the frequency of operation of the induction motor.
This type of HF tools made their appearance in the market a few decades ago. As the tools required higher frequency, there was a centralized frequency converter to convert the 50 or 60 Hz, three-phase supply to 200/300/400 Hz three-phase supply. There were separate running power lines to distribute the HF supply to various places in the shop floor. Non-standard electrical accessories in the form of plugs and sockets were used to differentiate them with the standard parts meant for 50/60 Hz usage.
While the HF tools were advantageous from the point of view of reliability, productivity and operational efficiency, they suffered in terms of high cost of installation of the centralized HF converter and distribution system. They were virtually excluded in the one-off usage or DIY applications due the high costs of the high frequency converter and the distribution network. They found their use only in cases where a battery of such tools is applied. Even here, there was the disadvantage that the HF converter had to be switched ON even when only one or a few tools were needed to be operated.
Also both the HF and the pneumatic tools suffer from the handicap that they are not truly portable in the sense that a separate air line or HF line is required for operation of them. And they certainly ruled themselves out in the case of DIY or typical one-off usage in smaller shops.
One of the objects of this invention is to obviate the above drawbacks by utilizing the electronic circuit of a frequency cum phase inverter, as described in my co-pending U.S. patent application filed concurrently for an AC motor or a brushless DC (BLDC) motor within the tool itself, which is incorporated herein by reference.
The second object of the invention is to provide heat sink for the power transistors of the PWM bridge inverter of a high frequency cum phase inverter in the tool.
The third object of the invention is to accelerate the motor in a soft-start mode limiting the in-rush current during starting.